Re: Help for new Q-200 pilot/owner


David J. Gall
 

[WARNING: Long post]
Joe,

My comments/opinion has been available for several years:
http://david.gall.com/files/Airplane/quickie1.txt.

A large part of "The Problem" is that non-engineers tend to think in terms
of "stability and control" whereas engineers know that "stability" and
"control" are two separate but related issues. You can overcome inadequate
stability with a more effective control if you have intelligence in the
system to operate the control appropriately. Better is to repair the
inadequate stability.

Common lore has it, and it can be shown through analysis, that a taildragger
is directionally unstable unless the tailwheel is firmly planted on the
ground. That's all fine and dandy, but such "analyses" typically ignore
numerous important factors that impact the directional stability (and the
control authority) available. For example, if the tailwheel is
free-swivelling and there's no pilot input, you might as well not have the
tailwheel on the ground. Thus, the common lore is incomplete in that it
fails to mention that there must be some centering force or a lock on that
tailwheel for it to impart directional stability.

Likewise, a big vertical stabilizer can give all the directional stability
needed without the tailwheel being on the ground, but you'd better have an
adequately sized rudder to compensate for any crosswind before relying on
the vertical stab/rudder combo to give you tail-up directional stability.
Thus, the rudder size and hence the control authority of the rudder plays
into the stability question.

Your question goes to the heart of a little-understood and almost completely
ignored aspect of the problem: the effects of camber and toe on directional
stability and control.

Negative camber and negative toe are destabilizing. With a rigidly locked
tailwheel, the typical taildragger (Pitts or Quickie, it doesn't matter) can
be made to be directionally unstable or directionally stable depending on
the choice of camber and toe settings, and the geometry/flexibility of the
landing gear. A bigger rudder will NEVER be able to fix that, because a
bigger rudder does not fix the alignment. But a bigger rudder control can be
used to OVERPOWER the destabilizing effect of improper camber/toe settings.

With improper alignment, stability deteriorates with increasing ground
rolling speed. Rudder authority increases with increasing airspeed. So long
as the airplane is operated in thick air (low density altitude) with
headwinds (airspeed greater than groundspeed) and at or below gross weight
(liftoff at a reliably "low" ground rolling speed), the increased rudder
size Mr. Postma advocates for the Quickie can probably reliably overcome the
directional instability inherent in the design.

So can lots of other things, like planting the tailwheel more firmly on the
ground through use of reflexor and moving the main wheels farther forward
and pilot technique with the pitch control. Also, additional "leverage" on
the tailwheel steering control through proper gearing to the rudder pedals
(belcrank mod), use of a better tailwheel and tire, the addition of
conventional tailwheel control springs, and the addition of independent
brakes (toe or finger) can all give more control authority to overcome the
inherent directional instability of the design. That's the Jim-Bob six-pack.

But wouldn't it just be simpler to "fix" the inherent instability in the
first place? Anything you do to the control system to enhance control
authority will still be welcome so long as it doesn't make the airplane too
sensitive, but sensitivity to controls ("twitchiness") is a symptom of
instability, so no worries if we fix the stability AND have all those
goodies from the Jim-Bob six-pack.

"Fixing" the stability by adjusting the camber and toe is so simple that
many people have already done so with good results reported by all. If you
build the airplane using the plans, you end up with 5 to 6 degrees of
negative camber at gross weight; "fixing" this amounts to simply setting the
camber to near zero at gross weight; that's all that is required. Even
getting half-way back to zero camber is beneficial.

I don't even entertain the debate about toe-in vs. toe-out, because it is so
insignificant in light of the gains to be had from proper camber. Set the
toe a little bit out (positive) in accordance with the plans if you wish.
You can also go a little bit positive on the camber without ill effect. Even
if you pooh-pooh the stability benefits of proper alignment, tire wear is
reduced because the tires are more closely perpendicular to the pavement
and, even with the plans amount of toe-out, almost parallel to the direction
of travel.

You take your car for a front-end alignment, why shouldn't you do the same
for your airplane? However, simply "fixing" the alignment is not enough for
the Q2/200. There are still important safety issues to be resolved by
employing the Jim-Bob six-pack. The canard is apparently sufficiently
flexible to be a contributor to instability in itself, and the stock
tailspring is prone to breakage. There are other, more subtle issues that
the six-pack addresses so I'll just touch on them briefly:

The reflexor is apparently necessary as a flight control as well as a
landing aid; the independent brakes are a "must" for the obvious reasons
with the wheels way out on the canard tips, as well as being available as a
secondary directional control if the rudder/tailwheel control circuit breaks
for any reason (most commonly a broken tailspring if the stock one is used).
Of course, the "LaRue brake mod" makes it possible for the brakes to work
correctly, so it is part of the six-pack, too.

The belcrank in the tailcone serves multiple purposes, although some clever
individuals have thought their way around using it. It reduces loads on the
lower rudder bearing while giving a sturdy place to absorb both the pilot's
control input loads and the tailwheel control cable loads (pavement
vibrations, etc.). It also permits the correct "gearing" of the tailwheel to
the rudder; tailwheel control deflections need only be a small fraction of
the rudder deflection for any given desired amount of control input,
although the gearing is not so obvious at the belcrank as that statement
would imply because use of tailwheel control springs adds some gearing
effect, too.

Tailwheel control springs are needed to permit the control input to be
converted into a pressure against the tailwheel instead of a definite
deflection, because the tailwheel is bumping along and needs a certain
amount of free play while still exerting a steering force against the
ground. Finally, the stock tailwheel that came in the kits sucks, so it
needs to be replaced with a proper tailwheel unit. This also changes the
geometry of the tailwheel to one that works more correctly with the
tailwheel control springs, and changes the tail spring to one that is far
less likely to break.

So, once the camber and toe are adjusted correctly so that we have some
common baseline of airplane directional stability from which to work, and
once the Jim-Bob six-pack has been installed so that we're all on a level
playing field, then we can discuss the RELATIVE merits of a larger rudder.
Until then, all comparisons are moot because there's nothing RELATIVE about
such disparate airplanes as what we campaign. Not disparaging here, after
all, EXPERIMENTAL is the name of the game, but there's really so little
common ground for comparative analysis when there are seven or eight major
variables just among the camber/toe and Jim-Bob six-pack.

To "debate" an increase in rudder size (a ninth variable) is ludicrous. To
"discuss" it, on the other hand, is quite welcome and absolutely what this
forum and the Q-Performance forum is all about.

Now, where was I...?

Oh, yeah, my two cents worth: I vote against the rudder size increase as
unnecessary, but I fully support Mr. Postma's advocacy thereof. The only
drawback I see is a potential for flutter, but the belcrank mod goes a long
way toward taking the slack out of the rudder control circuit and reducing
that potential....

The foregoing is entirely the opinion of the author, who has not built or
flown a Quickie or Q2/200 (yet!).


David J. Gall

-----Original Message-----
From: joseph.m.tygart.alion@...
[mailto:joseph.m.tygart.alion@...]
Sent: Tuesday, November 23, 2004 6:20 AM
To: Q-LIST@...
Subject: RE: [Q-LIST] Re: Help for new Q-200 pilot/owner


Q-Group - Any comments, opinions or observations on Q2 main
gear "toe in/out" and camber? Thanks,



Joe (N672CP)

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